[Rcpp-devel] Deep copying a matrix

[Dirk Eddelbuettel]

On 14 July 2011 at 12:36, Etienne B. Racine wrote:
| I'm struggling with the copy of my parameters. If I understand correctly, if
| you ship an int matrix to the function, you'll get a deep copy. However if it's

Wrong.

As Doug alluded to, you need the .clone() method for (intentional) deep
copies.  In other words, but default all copies are what one could call
shallow, in fact we simply access the same SEXP structure you had in R.

| a numeric matrix, then there's no copy and you get a matrix pointing on your R
| matrix. That's funny when you try to debug as one might work and the other not.

It basically boils down to user error on your side, and our inability to do
much about this code-wise.  Let me recap:

| Here's an illustration
| library(Rcpp); library(inline)
| 
| f1 <- cxxfunction(signature(origin="numeric"), body = "
|                   Rcpp::NumericMatrix m_source(origin);

So you want a _numeric_ matrix but ...

|                   m_source(0,0) = 0;
|                   cout << origin << endl << m_source << endl ;
| ",
|                   include = "using namespace std;",
|                   plugin="Rcpp")
| 
| m <- matrix(1:4,2,2)

you send in an _integer matrix_ and because the types do not mix, we must
copy. This is a little esoteric, but we discuss that in one or two places and
it has come up here too 

| x <- f1(m)
| 
| #0x9c7fe18
| #0xb416a1a8  # different

Because of the int != numeric issue.

| m
| #     [,1] [,2]
| #[1,]    1    3
| #[2,]    2    4
| m <- matrix(1:4/2,2,2)
| x <- f1(m)
| #0xb416a090
| #0xb416a090  # no different

Because numeric == numeric.

| m
| #     [,1] [,2]
| #[1,]    0  1.5
| #[2,]    1  2.0
| 
| 
| Now, what I'd like is to have a (deep) copy of that matrix, whatever the type.

.clone should do that.

Hth, Dirk

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